1. Field of the Invention
The present invention relates to a toner for developing electrostatic images which is used when an image is formed by an electrophotographic method or an electrostatic recording method, a developer for developing electrostatic images, a process for preparing a toner for developing electrostatic images, and an image forming method using the developer for developing electrostatic images.
2. Description of the Related Arts
A method for visualizing image information via an electrostatic image such as an electrophotographic method is utilized currently in a variety of fields. In an electrophotographic method, an electrostatic image is formed on a photosensitive material by an charging step and an exposing step, and an electrostatic latent image is developed with a developer containing a toner, which is visualized via a transferring step and a fixing step. As a developer used herein, there are a two-component developer composed of a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner alone. As a process for preparing a toner, a kneading and grinding process is usually used in which a thermoplastic resin is melt-kneaded with a pigment, an charge control agent, and a releasing agent such as a wax, the mixture is cooled, finely-divided, and classified. If necessary, an inorganic or organic fine particle is added to the surface of a toner particle in some cases in order to improve the flowability and the cleanability.
In a conventional kneading and grinding process, since a shape of a toner and a surface structure of a toner are irregular and subtly changed depending on the grinding property of materials to be used and conditions of a grinding step, it is difficult to intentionally control a shape and a surface structure of a toner. In addition, in the case of a material having the particularly high grinding property, a mechanical force in a developing machine frequently leads to occurrence of a finer powder and change in a shape of a toner.
Due to these influences, in a two-component developer, deterioration of electrification of a developer is accelerated due to adhering of a fine powder to the surface of a carrier and, in a one-component developer, toner scattering occurs due to extension of a particle size distribution, and deterioration of the image quality easily occurs due to reduction in the developability derived from change in toner shape.
In addition, when a releasing agent such as a wax is added into a thermoplastic resin to formulate into a toner, a releasing agent is exposed on the surface of a toner in many cases depending on a combination with a thermoplastic resin. In particular, a toner comprising a combination of a resin which is given the elasticity by a high-molecular component and is slight difficult to grind, and a fragile wax-type releasing agent such as polyethylene, polyethylene is exposed on the surface thereof in many cases.
Although such the exposure is advantageous from the viewpoint of the releasability at fixing and cleaning of an untransferred toner from a photosensitive material, polyethylene present in a superficial layer of a toner is easily transferred to a developing roll, a photosensitive material, a carrier or the like by a mechanical force, to easily contaminate them, leading to reduction in reliance. In addition, since a toner shape is indefinite, even when a flowing assistant is added, the flowability can not be sufficiently maintained, a fine particle on the surface of a toner is transferred to a concave part by the action of a mechanical force during use, whereby, the flowability is reduced with time, a flowing assistant is embedded into the interior of a toner and, thus, the developability, the transferability and the cleanability are deteriorated.
In addition, when a toner recovered by cleaning is reused by returning to a developing machine, the image quality is easily reduced. When a flowing assistant is further increased in order to prevent these, it comes in the state where occurrence of a black spot on a photosensitive material and scattering of flowing assistant particle are caused.
Recently, as a method of intentionally controlling a toner shape and a surface structure, there has been proposed a process for preparing a toner by an emulsion polymerization aggregating method (for example, see Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439). Since the emulsion polymerization aggregating method usually uses a finely-divided raw material of 1 micron or smaller as a starting material, a small size toner can be effectively prepared in principle.
More particularly, the emulsion polymerization aggregating method is a process in which, in general, a resin dispersion prepared by emulsion polymerization and the like, and a colorant dispersion in which a colorant is dispersed in a solvent are mixed, to form an aggregated particle having a diameter corresponding to a toner particle diameter and, thereafter, the aggregated particles are coalesced together by heating to obtain a toner. However, since the surface and the interior of a toner usually become to have the same composition when following this method, it is difficult to intentionally control the surface composition.
In order to solve this problem, there has been proposed a means which enables more precise control of a particle structure by freely controlling layers from an inner layer to a superficial layer of a toner upon preparation of a toner by the emulsion polymerization aggregating method (see JP No. 3141783). Since this process can easily prepare a toner having a small diameter and can realize precise control of a particle structure, it has become possible to realize both remarkable improvement in the quality of an electrographic image and high reliance.
On the other hand, recently, an image forming method by electrophotography using the above-described toner and developer technology has begun to be applied to a part of a printing field with progress of digitalization and coloration, and has begun to be remarkably put into practice in the graphic art market including on-demand printing.
The graphic art market refers to a total business market relating to producing prints, such as copying and duplication of created print which is to be copied at a small copy number such as woodcut print, and of originals such as, handwriting and picture, and mass production printing called reproduction, and is defined as the market targeting industry associated with production of prints.
For example, in the short run printing market, not only monochromic printing utilizing the characteristics of plateless printing in electrophotography, but also the technologies targeting the short run color market, a representative of which is Fuji Xerox Co., Ltd. Color DocuTech60 have been developed, and have greatly progressed from the viewpoint of the image quality, the paper adaptability, the product price and the price per paper (Bulletin of Japan Image Society, Vol. 40 No. 2 2001).
However, when compared with original genuine conventional printing, although the on-demand characteristics as plateless printing are present, there arise deterioration of color reproduction region, resolution, image quality represented by the glossiness, the texture; deterioration of image uniformity in a single image, and the retainability of the image quality at continuous printing for a long period; high price per paper due to an amount of a consumed toner at a high image density; deterioration of the adaptability to a thinner paper and a thicker paper; the image defect and the deteriorated writability caused by a presence of an oil at image fixing; and high consumption of electric power at high speed high temperature fixing; elongation, curling, and waving of a paper due to image fixation at a high temperature and a high pressure, and a discrepancy in a register mark at printing on both surfaces, being problematic.
In addition, in principle, since a toner image comprising a low-molecular resin having a relatively low softening point is thermally fixed, there is a case where the thermal or mechanical durability of an image is weaker than that of a printed image. When a paper is bent multiply, or papers are subjected to bookbinding, piled multiply and exposed to a high temperature under the high load state, there arise problems on defect of an image, and on resistance to various stresses such as the light resistance and the weather resistance accompanied with blocking, offset and outdoor exposure, in some cases.
It has been found, in order that an image formed by electrophotography like this considerably replaces the previous printed material and pursues the market value, in particular, as a production means in the graphic art region, there are still a number of problems to be solved.
In addition, regarding a color reproduction region, kinds of pigments which have been put into practice in an electrophotographic field are smaller than kinds used in the previous printing ink, and the technologies for further high performance colorants are necessary. Since use conditions in the graphic art region are diverse as compared with the office market, not only the high color reproductivity, but also a variety of durabilities of an image such as the heat resistance, the light resistance, the waterproofing, the oilproofing, the resistance to solvent, the resistance to scuffing and the bending strength are required.
The resolution is easily restricted by a particle diameter of a toner and a distribution thereof as well as an image processing system, a photosensitive material and an exposure. There is the great technical problems in using a small size toner effectively and at a high reliance in respective processes such as electrification, development, transfer, fixing and cleaning.
Such the technical problems lie in, for example, a carrier for uniformly charging a small size toner, design of an charging blade and an charging roll, a developing system for obtaining the high image density without causing high background density, a transfer system for realizing precise transfer with high transfer efficiency, a fixing system for handling a combination of a small size toner and various papers, and a cleaning system for completely removing a small size toner from a photosensitive material or an intermediate transferred material, to realize a stable image quality.
In order to improve in-plane uniformity and the defects of an image, control of uniformity of the developing ability of a developer in an image forming system becomes important. In order to cope with the retainability of the image quality required in the printing market, there is required a highly durable developer which shows the stable charging property even in continuous printing of several thousand prints, maintains stable and uniform development, and is less dependent on the environment such as temperature and humidity. In addition, a developing system is required to avoid influence of a paper powder and foreign matters, be highly durable, suppress occurrence of defect and noise, and maintain the in-plane density uniformly.
In a system for performing transfer from a photosensitive material or an intermediate transferred material to a recording medium, an electrostatic transfer system is general in the current electrophotographic technology. In this electrostatic transfer system, in the case of a color image in which a thickness of an image of a toner becomes great by color overlapping, in order to suppress deterioration of an image due to toner scattering in transfer, optimization of a toner material and a transfer system for controlling the behavior of a toner in the electric field precisely is necessary. In addition, occasionally, a transfer system which can radically suppress toner scattering and which is not based on an electrostatic force such as adhering transfer becomes necessary.
As a cleaning system, it is important to optimize a system for cleaning a toner with a controlled shape such as a small diameter and a spherical shape at a high reliance and continuously without the environmental dependency by a method such as a blade, an electrostatic brush, a magnetic brush, web and cleaning simultaneous with development, also from a view point of optimizing a toner material, toner structure, and a hard system, as well as a highly durable photosensitive material.
For the cost per paper, it is necessary to reduce an amount of a consumed toner by a smaller diameter of a toner and optimization of an amount of a colorant. However, this easily influences on the uniformity of the image quality. For reducing the cost per paper, it is actually important to reduce “printed paper to be discarded” (waste output for obtaining the stable image quality) having the great influence on the price in printing, or reduce the maintenance load, by realizing an image forming system having a high reliance by the above-mentioned means.
In order to cope with the case where a material is a thin paper or thick paper, a toner material is required to have a property that, even when a material is a paper having little stiffness such as a thin paper or a plastic film, it is easy to peal a recording medium from a fixing members such as a fixing roll after fixation, and even when a material to be printed is a coated paper or a thick paper, low temperature fixation is possible in which an electric power consumed amount can be suppressed at the fixation.
Fixation at a low temperature and a low pressure can reduce a stress on a material, can suppress elongation, curling and waving of a material, and can overcome a problem such as a register mark discrepancy. In order to avoid the image defect and the deteriorated writability such as stain and streak due to an oil, an oil-less fixing apparatus and oil-less toner containing a releasing agent in the interior thereof become necessary.
In addition, in order to realize the image durability which is comparable to the usual printed image and does not cause a problem under various use circumstances, the property of a resin used in the previous toner must be further improved considerably.
In order to make the glossiness of an image to have a higher degree of freedom and to be uniform, control of the viscoelasticity of the toner and optimization of a fixing apparatus are important. In order to obtain an image having a high quality based on offset printing, realization of the optimal glossiness corresponding to a paper to be used is important in order to potentiate the market value, and optimization of a toner, a paper and a fixing system is necessary.
Further, the characteristic which is sought recently in the field such as on-demand printing is the environmental load performance. By on-demand printing business with network, the inventory can be eliminated or minimized. Thus, the environmental load accompanied with inventory, movement and waste thereof can be reduced, which is easily generated in the usual printing.
In addition, since a dry toner used in usual electrophotography does not use an organic solvent which is used in an ink used in a usual printing machine, the environmental load accompanied with VOC can be fundamentally reduced. For further improvement, not only reduction in the electric energy accompanied with fixation of an image and maintenance of condition of a hardware, but also reduction or nonuse of odor or volatile substance in a heated and melted resin generated at the fixation, and suppression of discharge of a small size toner component from a machine, are important subject. In addition, it is necessary to consider the recycling property of a wasted toner and a printed paper.
Like this, in order to fulfill the demand in the graphic art market and the short run market (which may be also referred to the simple printing market), the technology which is obtained by developing the previous electrophotographic technology further highly as a system, becomes necessary.
In such circumstances, in order to realize the higher image quality, design of the property of a resin becomes extremely important.
In order to realize a wide color reproduction region, it is necessary not only to optimize a color material but also optimize the melting property of a resin to obtain an image having a glossiness higher than a certain level. For doing so, the property of a resin is designed so that since the elasticity of a resin contained in a toner reduces upon heating of a toner by a fixing roll at the fixation, the melt viscosity reduces and the flowability of a resin increases. Therefore, in this case, it is necessary to reduce a molecular weight of a resin.
However, when an elasticity of a resin is reduced, the attachability between a fixing roll and a toner at the fixation is enhanced and, even when a releasing agent such as a wax is contained in a toner, it is difficult to peal the toner from a fixing roll in the state where an oil is not imparted on the surface of a hot roll. In addition, a toner having a lower molecular weight easily causes occurrence of hot offset at a high temperature, resulting in a toner having an extremely narrow usable temperature at the fixation.
When such the toner is used, since the fixing behavior becomes extremely sensitive to lowering of a temperature of a fixing roll at continuous printing and elevation of a temperature at heating of a heater built in a fixing roll, it becomes extremely difficult to control a temperature. Actually, since temperature influence due to the quality and a thickness of a paper is added to these factors, the control is further complicated.
In addition, when a molecular weight of a toner is lowered, even when the glossiness is increased, an image becomes fragile mechanically, the image defect is easily caused by folding of a recording medium such as a paper, and a problem is easily generated from the viewpoint of the durability of an image.
In addition, recently, in view of the environment, decrease in an amount of the energy to be used at the fixation, that is, decrease in a fixing temperature is desired. In order to lower a fixing temperature, it is effective to lower a glass transition temperature of a resin. However, when image formation, the retainability of an image after fixation, and the durability of the image are taken into consideration, it is difficult to use a resin exhibiting a low glass transition temperature as a material constituting a toner.
Further, an image formed by electrophotography as on-demand printing has recently been spreading as a substitute for simple printing. On the other hand, an image utilized in such a use is left for a long time under the environment at a high temperature and a high pressure when stored in a warehouse after bookbinding and piling in distribution process in many cases. When a printed material is left for a long time under the environment at a high temperature and a high pressure, an image formed by an electrophotographic method easily causes blocking of a printed material, and is remarkably inferior in the thermal durability as compared with an image printed with a usual printing ink. For this reason, a utilization field of an image formed by an electrophotographic method is limited. Further, when the glossiness is desired to be enhanced in order to obtain a high color region as described above, a problem on the thermal durability of the image has become more remarkable.